HATCHING. 



35 



~-7IW> 



inp 



Fig. 18. — Longitudinal-vertical 

 section tlirough an embryo of 

 Amphioxus with the rudi- 

 ments of two somites (after 

 Hatsehek). nifl pole-cells, the 

 existence of which is now 

 denied ; mr uncovered part of 

 medullary plate ; mr^ space 

 between the medullary plate 

 plate and overgrown ectoderm 



,. (future medullary canal ); 

 us', Its" mesoblastic somites. 



and generative organs in a manner de- 

 scribed below. The collar-somites send 

 forward a process dorsal to the head 

 cavities on each side, the walls of 

 which give rise to the mesodermal 

 structures of the preoral region. 



While these changes have been tak- 

 ing place the notochord is developed. 

 It arises as a groove of the dorso- 

 median endoderm (Fig. 17 B), which 

 is constricted off so as to form a solid 

 rod of cells lying between the dorsal 

 endoderm and the medullary plate. It 

 develops from before backwards, ex- 

 cept the front jDortion, wliich separates 

 from the endoderm later than the 

 rest and extends to the anterior end 

 of the body. 



The embryo becomes hatched and 

 begins to swim freely in the sea by 



means of the cilia of the ectoderm cells at about the stage of 

 Fig. 18, about twelve hours after fertilization. But it remains 

 opaque and, being incapable of taking in food, is generally spoken 

 of as an embryo until about the thirty-sixth hour, when the yolk 

 is sufficiently absorbed to leave the tissues transparent, and the 

 mouth, anus, and first gill-slit are formed (Fig. 20). 



The mouth is 

 formed on the 

 left side ; the 

 first gill-slit on 

 the ventral 

 middle line, soon 

 moving on to 

 the right side, 

 and the anus at 

 the hind end of 



Fig. 19. — Diagrammatic lonfjitudinal section through an embryo "^"^ DOdy Sllglltly 

 of Amphioxus, to show the formation of the coelomic sacs. j. j.! I^^ff ^f flip 

 The .figure combines features which in reality would only be ^"^ iii^ti leit OI lliv 

 shown by two sections. 1 neuropore ; 2 neural canal ; 3 neuren- 

 teric canal ; 4 coelomic groove ; 5 somite constricted off from 

 anterior end of coelomic groove ; 6 cavity of first mesoblastic 

 somite (so-called collar cavity) ; 7 head cavity ; 8 archenteron. 



middle line. 

 The larval 



